The present invention relates to a pressurized water nuclear reactor instrumentation system for determining the three-dimensional nuclear power distribution in the nuclear reactor core. More particularly, the present invention relates to a device for use with an in-core instrumentation system to measure the intensity of radiation induced in an activated movable member of the instrumentation system.
It is well known, that the three-dimensional nuclear power distribution within a reactor core can be inferred by employing an activation type incore instrumentation system. Such a system measures the intensity of radioactivity induced in an activated member such as long wires or columns of small balls. The long wire or balls such as hydroballs or aeroballs (i.e. an activated member) are introduced into the reactor core via tubes or instrumentation thimbles. The instrumentation thimbles extend for the length of fuel assemblies within the core. One such system is described in a co-pending U.S. patent application having U.S. Ser. No. 07/042,183, entitled Hydro-Ball In-Core Instrumentation System and Method of Operation which is assigned to the same assignee as this application. This copending application, U.S. Ser. No. 07/042,183, is hereby incorporated by reference (hereinafter "'183 application").
In the '183 application, a string of balls is introduced into the reactor core and positioned at sensing positions where they are irradiated. The irradiated balls are then retracted and the intensity of gamma rays emitted by the radioactive balls is measured. However, gamma rays have relatively long mean free paths, even in the most effective shielding materials. As a result, it is extremely difficult to measure the intensity of gamma ray radiation from an individual ball because the measurement is contaminated by gamma ray radiation from adjacent balls. This contamination or cross-talk blurs the measurement of gamma ray radiation intensity of each ball.
It is known that beta rays (i.e. energetic electrons emitted in nuclear decay) have much shorter mean free paths than do gamma rays (i.e. electro-magnetic radiation). Proper shielding of measurement apparatus is therefore more easily accomplished when detecting beta rays rather than gamma rays; reducing contamination (e.g., cross-talk or blurring) of measurements of emissions from an irradiated member such as a ball or wire. Kraftwerk Union AG of Mulheim on Ruhr in the Federal Republic of Germany has incorporated a beta ray detection device in a system that relies on irradiation of movable detectors (i.e. aeroballs) to measure power distributions in pressurized water nuclear reactors. However, in the Kraftwerk aeroball system, the instrumentation thimbles within the reactor vessel form part of the reactor coolant system pressure boundary, isolating the entire aeroball transport and counting system from the reactor coolant. The environment within the aeroball system is that of a gas, typically dry nitrogen, at approximately atmospheric pressure. In this environment use of a conventional beta ray counter, in which ionization of a gas by emitted beta particles is monitored, is feasible and provides a measure of the induced radioactivity of the irradiated aeroballs.
The hydroball system considered here cannot admit a conventional beta ray counter, as does the Kraftwerk system, since the environment within the hydroball system is a liquid at high pressure and so requires a pressure boundary that is too thick-walled for any emitted beta rays to penetrate through to an external beta ray counter. If the benefits of beta ray counting are to be realized in a hydroball system, the radioactivity measuring device must be capable of functioning in a liquid medium and must be installed inside the system pressure boundary.